The invention relates to a limited slip differential system for the drive train of a motor vehicle with continuous all-wheel drive, having an intermediate planetary transmission in the direct through drive and a corresponding braking means engageable with the intermediate planetary transmission in response to the rotational speed difference between the front and rear axles of the motor vehicle.
An all-wheel drive is described in German Published Unexamined Application No.(DE-OS) 32 18 830, in which a transfer transmission constructed as a planetary transmission is connected behind a speed change transmission for distributing the driving load between the front axle and the rear axle of the motor vehicle. A drive branch of the planetary gear transmission includes a switchable clutch and a switchable brake which selectively connect a driving branch to the transmission housing. The gear ratio of the planetary transmission is chosen such that when one driving branch is disconnected by disengagement of the clutch and the application of the brake, the other driving branch will be driven with a transmission. In this system, under good road conditions, driving with a splitting of the entire transmission and quick gear shifting are both facilitated, while the front and rear axles can be driven when the clutch is engaged and the brake is disengaged.
An object of the present invention is to provide a limited slip differential system of the above-described type, which upon slipping of one of the axles, correspondingly limits the rotational speed difference with respect to the other axle. Therefore, synchronous running of both axles is facilitated, without requiring a high braking moment and a large volume brake.
This and other objects are attained by providing that a limited slip differential system of the above-described type has a substantially higher transmission ratio between the braking device and the planetary transmission than the transmission ratio for the direct through drive.
An especially preferred embodiment of the present invention provides that the gear ratio of the inlet sun gear wheel to the planetary carrier which is connected to a braking member of the braking device, is 16:1. Therefore, a resistance to the transfer of rotational speed of the driven shaft to the rear axle is created, which results in a rotational moment transfer to the rear axle. Due to the high gear ratio and the low associated toothed-gear wheel efficiency, the required braking moment is very small so that only a small volume hydrodynamic or centrifugal brake is needed.
An advantageous feature of a preferred embodiment of the present invention is the use of a brake with progressive braking moment to axle rotational speed difference characteristics. With this feature, the transfer moment for small slip values is also kept small. Thus, the vehicle exhibits good handling, and an ABS system will be fully functional. The transfer moment rises very steeply with large slip values, which can occur, for example, if the wheels of one axle are on ice and the wheels of the other axle are on grippable asphalt. The steep rise of the transfer moment in this situation allows problem-free driving.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, embodiments constructed in accordance with the present invention.